1. Technical Field of the Invention
The present invention relates to wireless local loop communications systems and, in particular, to a ring topology configured wireless local loop system possessing with a communications fallback procedure to an overlaid cellular communications system for wireless local loop subscribers in the event of a local loop system failure.
2. Description of Related Art
Reference is now made to FIG. 1 wherein there is shown a functional block diagram of a wireless local loop arrangement 100 using a wireless Digital Subscriber Line Access Multiplexer (W-DSLAM or a xe2x80x9cradio headxe2x80x9d) module 102. A high-speed common carrier network 104 is connected from one of its Asynchronous Transfer Mode ATM switches 106 to the W-DSLAM module 102 over a high-speed wireless link (aggregate path) 108. The aggregate throughput provided via the wireless link 108 is approximately 40.0 Mbps. The W-DSLAM 102 functions to multiplex this aggregate throughput into a plurality of wireless distribution channels 110. Each wireless distribution channel 110 is capable providing a throughput of between (approximately) 384 Kbps and (approximately) 2 Mbps. At these operating throughput rates, the W-DSLAM 102 is capable of concentrating a substantial number of channels into the single high speed wireless aggregate path link 108. Each of the wireless distribution channels 110 is terminated with a network termination (NT) node or module 112 that is provided at a user""s (or subscriber""s) home, office, business, or any other facility. The network termination module 112 may further operate in a multiplexing fashion to allow plural devices (D) 114, each requiring a portion of the bandwidth provided by the wireless distribution channel 110, to be connected for communication. Examples of such devices 114 include: a computer, a telephone, home appliances, office devices, and microprocessor controlled components and the like.
The high-speed common carrier network 104 is provided as a Synchronous Optical Network (SONET) or related Synchronous Digital Hierarchy (SDH) ring 120 formed from a plurality of cable interconnected ATM Add/Drop Multiplexers (ADM) 122. Other network elements, such as a server 124, routers 126, and ATM switches 106 are interconnected to the ring 120. The ATM switches 106 connecting with the W-DSLAM 102 over the link 108 comprises a point of presence (POP) node for providing wireless local loop arrangement 100 access to the common carrier network 104. This POP ATM switch 106 is preferably connected to one of the ring ATM/ADM modules 122 via a high-speed link 130 capable of throughput in the order of several hundred megabits per second. As one example, the high-speed link 130 may be implemented to transport an Optical Carrier 12 (OC-12) optical signal with a throughput of around 622.080 Mbps.
The wireless local loop distribution channels 110 may advantageously utilize the Digital Subscriber Line (DSL) modem technology to support transmission operating rates that are capable of delivering a host of broadband applications such as multimedia, video-teleconferencing, video-on-demand, games-on-demand, and the like. Preferably, a suitable Time Division Multiple Access (TDMA) scheme may be utilized for the air interface required for implementing these distribution channels 110. Further, the air interface may be provided in accordance with standard protocols such as ANSI-136 or GSM, using a common carrier signal, for example, a 200 KHz (for a 384 Kbps data rate) or 1.6 MHz (for data rates up to 2 Mbps) signal.
A wireless local loop communications system is provided wherein a plurality of digital subscriber line access multiplexers are interconnected with each other in a ring topology via a plurality of first wireless communications links. Each included network terminal providing local loop access to a connected subscriber device is connected to an associated one of the digital subscriber line access multiplexers via second wireless communications link. The first and second wireless communications links support communication transmissions at a first and second throughput rate, respectively, wherein the second throughput rate is less than the first throughput rate.
The wireless local loop system is overlaid by a conventional cellular communications system. The cellular communications system includes a base station in each cell capable of supporting wireless communications with cellular mobile stations over an air interface. In the event of a failure in the wireless local loop system that would leave a certain network terminal stranded with respect to its communications service, that stranded network terminal instigates a mode shift from use of the second wireless communications link to instead utilize the cellular system air interface for its subscriber device communications. The air interface supports communication transmissions at a third throughput rate that is less than the second throughput rate, and accordingly the communication being handled by the stranded network terminal has its throughput adjusted with the mode shift to match the cellular air interface supported throughput.